INTRODUCTION: SOUND IS A FORM OF ENERGY WHICH PRODUCES A SENSATION OF HEARING IN OUR EARS. Production of sound Sound is produced when a body vibrates, i.e. sound is produced by vibrating bodies. Activity 12.1 § Take a tuning fork and set it vibrating by striking its prong on a rubber pad. Bring it near your ear. § Do you hear any sound? § Touch one of the prongs of the vibrating tuning fork with your finger and share your experience with your friends. § Now, suspend a table tennis ball or a small plastic ball by a thread from a support [Take a big needle and a thread, put a knot at one end of the thread, and then with the help of the needle pass the thread through the ball]. Touch the ball gently with the prong of a vibrating tuning fork (Fig. 1). § Observe what happens and discuss with your friends. Activity 12.2 § Fill water in a beaker or a glass up to the brim. Gently touch the water surface with one of the prongs of the vibrating tuning fork, as shown in Fig. 2. § Next dip the prongs of the vibrating tuning fork in water, as shown in Fig. 3. § Observe what happens in both the cases. § Discuss with your friends why this happens. § § Important points: 1. Vibration means to and fro kind of motion of an object. 2. All vibrating bodies produce sound. 3. Sound cannot be produced if the body is not vibrating. Propagation of Sound: • • • • The matter or substance through which sound is transmitted is called a medium which can be solid, liquid or gas. When an object vibrates, it sets the particles of the medium around it into motion. A particle of the medium in contact with the vibrating object is first displaced from its equilibrium position which exerts force on the next particle and displaces it from its position of rest. After disturbing the next particle the first particle comes back to its original position and this process continues till the disturbance reaches our ears. Disturbance created in the medium travels through the medium and not the particles of the medium as they do not move forward themselves. Example • If we drop a stone in a pond, we see circular water waves (ripples) spreading out in all directions on the surface of water. If we place a small leaf on water surface, the leaf moves up and down about its original position but does not move away from or towards the source of disturbance along with the waves. A wave is a disturbance in a medium which carries energy from one point to another without there being a direct contact between the two points. • Wave motion is a vibratory disturbance produced in one part of the medium that travels to another part involving the transfer of energy but not the transfer of any matter with it. • Air is the most common medium through which sound travels. Sound needs a medium to travel:- • • • The waves which need material medium (solid, liquid or gas) for their propagation are called elastic or mechanical waves e.g. sound waves. Sound waves cannot travel through vacuum. Bell jar experiment to be discussed and explained using the Smart Class module. There are two types of Mechanical waves: Longitudinal waves and transverse waves. (Note :In addition, transverse waves include electromagnetic waves as well.) Longitudinal Waves In these waves the individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance. Compression is that part of a longitudinal wave in which the particles of the medium are closer to one another than they normally are and there is a momentary decrease in the volume of the medium (region of high pressure). Rarefaction is that part of a longitudinal wave in which the particles of the medium are farther apart than that in normal position and there is a momentary increase in the volume of the medium (region of low pressure). Longitudinal wave is represented by density-distance graph. Transverse Waves In these waves the individual particles of the medium move up and down about their mean positions in a direction perpendicular to the direction of wave propagation. Examples: Radio waves, light waves, waves in water, waves in string etc. Crest is the point of maximum positive displacement of a transverse wave. Trough is the point of maximum negative displacement of a transverse wave. Transverse wave is represented by displacement-distance graph. Differences between Longitudinal waves and Transverse waves (Mechanical) • Parameter 1.Vibrations of the particles Longitudinal Waves In these waves the individual particles of the medium move in a direction parallel to the direction of propagation of the disturbance 2. Regions formed They consist of regions of compression (region of high pressure) and rarefaction (region of low pressure). Transverse Waves In these waves the individual particles of the medium move about their mean positions in a direction perpendicular to the direction of wave propagation. They consist of crests and troughs representing the regions of displacement from the mean position. 3. Medium They are possible in all kinds of media(solids, liquids and gases) They are possible in solids and surface of liquids only. 4. Pressure and density variation The pressure and density are maximum at compressions and minimum at rarefactions. There is no pressure variation in the medium. 5.Examples Sound waves, waves produced in slinky when it is pushed or pulled. Light waves, waves produced in slinky when it is moved up and down. Characteristics of a Sound wave:1. Phase: The points on a wave which are in the same state of vibration are said to be in the same phase.(all points that lie on the crest-B, F and J are in the same phase, points D and H are in the same phase and A, E, I are in the same phase). 2. Wavelength: Wavelength (length of one complete wave):- The distance between two consecutive compressions (C) or two consecutive rarefactions (R) is called the wavelength. It is represented by. Its S.I. unit is metre (m). The distance between a crest and an adjacent trough is equal to half the wavelength (l). 3. Frequency: The number of complete waves produced in one second is called frequency of the wave. It is represented by υ (nu) and its S.I unit is hertz(Hz). 4. Time period: The time required to produce one complete wave is called time period of the wave. It is represented by symbol T and its S.I unit is second (s). 5. Amplitude: The maximum displacement of the particles of the medium from their original undisturbed positions, when a wave passes through the medium, is called amplitude of the wave. 6. Relation between Time period and Frequency No. of waves produced in Ts = 1 No. of waves produced in 1s = 1/T As no. of waves produced in one second is frequency, therefore, υ = 1/T Hz 7. Wave velocity: The distance travelled by a wave in one second is called the velocity of the wave. S.I. unit is m/s. Relation between Wave velocity (v), Frequency(υ) and Wavelength(l) Wave velocity = distance travelled by the wave Time taken v=d t Wave velocity = frequency x wavelength HOMEWORK: 1. Example 12.1 of Pg-166 2. A vibrating wire has a time period 0.05 s, calculate the frequency. 3. The frequency of a sound wave is 25 Hz. What is the time period? 4. A source produces 45waves in 5s. The distance between a crest and a trough is 15cm. Find a) frequency b) wavelength and c) velocity. 5. Calculate the time taken by a sound of frequency 500hz and wavelength 25cm to travel a distance of 100m. 6. The wavelength of the vibrations produced on the surface of water is 4cm. If the wavevelocity is 16m/s, calculate a) the no. of waves produced in 1s and b) time required to produce 1 wave. CHARACTERISTICS OF SOUND 1. Pitch: is that characteristic of sound by which we can distinguish a shrill sound from a hoarse sound having the same loudness. - It depends upon the frequency of vibration. - The faster the vibration of the source the higher is the frequency and higher is the pitch. -For example, voice of children and ladies is shrill as compared to the voice of men. 2. Loudness : is that characteristics by which a loud sound can be distinguished from a faint sound even though both have the same pitch. - It depends upon the Amplitude of vibration. -The greater the amplitude of vibration of the source, the greater is the intensity of sound and vice versa. 3. Quality: is that characteristics which enables us to distinguish one sound from another having the same pitch and loudness. -A sound of single frequency is called a tone. - Note is a sound produced due to mixture of frequencies. - Intensity of sound: The amount of sound energy passing each second through unit area. - Music is a pleasant sound and have a rich quality whereas noise is an unpleasant sound. Speed of sound in different media:Speed of sound depends on three factors:1. It depends on the properties of the medium through which it travels. 2. It depends on the temperature of the medium. The speed of sound decreases as we move from solid to gases and increases as we increase the temp of the medium. for ex speed of sound in air at 00 C is 331m/s at 220 C is 344m/s. The speed of light (in air 300000000m/s) is very great as compared to the speed of sound(in air 344m/s). Therefore the sound of thunder is heard a little later than the flash of light is seen. Reflection of Sound:- The changing of the direction of sound when it strikes a hard surface(like walls, metal sheets etc). • Like light, sound gets reflected at the surface of a solid or liquid and follows the same laws of reflection. Echo:- The repetition of the original heard after the sound is reflected from a distant, dense and rigid object (high building, cliff etc) of sound waves is called an echo. Conditions to produce an echo:• To hear a distinct echo the time interval between the original sound and reflected sound must be at least 0.1 s. speed of sound = 344 m/s time = 0.1s the total distance covered from the point of generation to the reflecting surface and back should be at least = 344 x 0.1 = 34.4m • The minimum distance between the source of sound and the reflector should be at least 17.2m (1/2 of 34.4m) • the reflecting surface should be hard enough. Reverberation:- The repeated reflection of sound that follow closely behind the original sound appears to be prolonged even when the source of sound stops producing sound. This effect is called reverberation. To reduce the reverberation :1. The roof and walls of hall are covered with sound absorbent materials like compressed fibre board, rough plaster or draperies. 2. Carpets are put on the floor to absorb sound. 3. Heavy curtains are put on the doors and windows. 4. The material having sound absorbing properties is used for making the seats in a big hall. Q1 on Pg-168 Q- A boy blew a whistle standing in front of a cliff. He heard the echo after 2s. Find the distance of the cliff from the boy if velocity of sound in air is 332m/s. Q - A girl hears an echo of her own voice from a distant tall building after 2s. What is the distance of the girl from the building? (Given speed of sound in air = 332 m/s) Uses of Multiple Reflection of sound:1. Megaphones or loudhailers, horns are designed to send sound in a particular direction, without spreading all around.. 2. Stethoscope is a medical instrument used by doctors for listening to the sounds produced within the human body, mainly in heart and the lungs. This also works on multiple reflection of sound. 3. Sound board is a curved board which is placed behind the speaker in large halls so that his speech can be heard easily even by the persons sitting at considerable distance. 4. The ceilings of big halls are curved so that sound after reflection reaches all corners of the hall. Range of Hearing:• The audible range of sound for human beings extends from about 20Hz to 20000Hz. • The sounds of frequencies lower than 20Hz are known as infrasonic sounds. e.g. sounds produced by vibrating simple pendulum, earthquakes, some animals like whales, elephants and rhinoceroses. Infrasonic sounds cannot be heard by humans. • The sounds of frequencies higher than 20,000Hz are known as ultrasonic sounds. Dogs can hear these sounds, therefore they are used for detective work by the police. Bats, monkeys, deer, cats, dolphins, porpoises & leopard can also hear these sounds.. What are Ultrasonic sounds? Ultrasonic sounds are high frequency sound having a frequency greater than 20kHz (inaudible range). Applications of Ultrasound:1. Ultrasound is used in industry for detecting flaws (cracks) in metal blocks without damaging them. 2. It is used in industry to clean ‘hard to reach’ parts of objects such as spiral tubes, odd shaped machines and electronic components etc. 3. It is used to investigate the internal organs of the human body such as liver, gall bladder, pancreas, kidneys, uterus etc. The technique of obtaining pictures of 4. 5. 6. 7. internal organs of the body by using echoes of ultrasound pulses is called ultrasonography. The use of ultrasound waves to investigate the action of heart is called ‘echocardiography. Ultra sound scans are used to monitor the development of foetus inside the mother’s uterus. It is used to break kidney stones into fine grains. It is used in ‘sonar’ apparatus to measure the depth of sea and to locate under sea objects like submarines, hidden icebergs in the sea. SONAR:- stands for ‘Sound Navigation And Ranging’ • is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects. • It consists of i) a transmitter for emitting ultrasonic waves ii) a detector for receiving reflected ultrasonic waves which is installed in a boat. The transmitter produces and transmits ultrasonic waves which travel through water and after striking the object on the seabed, get reflected back & are sensed by the detector. The detector converts the waves into the signals which are appropriately interpreted. Velocity of sound( in water) = 2 x distance (d) Time taken(t) Or, The distance of the object = (speed of sound through water x time taken 2 This is also called echo-ranging. SONAR technique is used to locate underwater hills, valleys, submarines, icebergs, sunken ship etc. Example 12.3, pg- 171 Q.1, pg-172 HOMEWORK Q- A sonar fitted in a submarine receives an echo 4s later the signal is transmitted towards another submarine 3060m away. Find the speed of sound in air. Q- An echo is heard in 5 s. What is the distance between the reflecting surface and the source of sound, given speed of sound is 344 m/s. Value Based Question: Mr. Kapoor was hard of hearing and was advised by the doctor to use hearing aid. He was not ready to listen to the doctor as he thought that it would cause harm to him and look odd. His friend told him that hearing aid is not harmful but would help him better in communicating with people and thus improve his personal relations at business and home to which Mr. Kapoor finally agreed. i) On what principle does the hearing aid work? ii) What values are shown by his friend? Quiz 1. When the vibrating object moves backwards, it creates a region of low pressure called — ————a. b. c. d. Refraction Reflection Rarefaction Retardation Answer: (c) Rarefaction Explanation: Rarefaction is the opposite of compression. Rarefaction means the reduction of density of the object. 2. Mexican wave in a stadium is an example of a. b. c. d. Longitudinal wave Transverse wave Electromagnetic wave None of the options Answer: (b) Transverse wave Explanation: Mexican wave, also known as the stadium wave, is an ideal example of a vertically polarised, transverse, travelling wave. 3. State true or false: Sound does not need a medium to propagate. a. True b. False Answer: (b) False Explanation: Sound needs a medium to propagate. The matter or material through which sound propagates is called a medium. 4. The distance which compression or a rarefaction travels per unit of time gives————a. b. c. d. The density of sound wave Speed of sound Wavelength of sound Frequency of sound Answer: (b) Speed of sound Explanation: Speed of sound measures the compression or a rarefaction that travels per unit of time. 5. Is the law of conservation of energy applicable to sound waves? a. Yes b. No Answer: (a) Yes Explanation: The law of conservation of energy is applicable to sound waves. 6. Sound travels through which medium? a. b. c. d. Solid Liquid Gas All the above Answer: (d) All the above Explanation: Sound has the ability to travel through solid, liquid and gas. 7. When a body vibrates, it compresses the air surrounding and forms a high-density area known as —————-. a. b. c. d. Refraction Reflection Rarefaction Compression Answer: (d) Compression Explanation: Compression is the opposite of rarefaction. Compression means increase in the density of the object. 8. The phenomenon where a sound produced is heard again due to reflection is called —— —– a. b. c. d. Sound bounce Mirage An echo Interference Answer: (c) An echo Explanation: To hear a distinct echo sound, the time interval between original and reflected sound must be at least 0.1s. 9. The number of compressions or rarefactions per unit time gives ———– a. b. c. d. Frequency Time period Amplitude Pitch Answer: (d) Pitch Explanation: The number of compressions or rarefactions per unit time defines pitch. The pitch is directly proportional to frequency. 10. Sound waves in air is an example of —————— a. b. c. d. Longitudinal wave Transverse wave Electromagnetic wave None of the options Answer: (b) Longitudinal wave Explanation: In longitudinal waves, particles travel parallel to the direction of wave motion employing successive compressions or elongations.
